Process of producing hard chromium platings on light metals



United States Patent PROCESS OF PRODUCING HARD CHROMIUNI PLATINGS 0NLIGHT METALS Paul Eyerund, Stuttgart-Weilimdorf, Germany, assignor toMetallgesellschaft Aktiengesellschaft, Frankfurt am Main, Germany, acorporation of Germany No Drawing. Application May 7, 1951, Serial No.225,065

Claims priority, application Germany June 16, 1950 8 Claims. (Cl.204-33) This invention relates to a process of producing hard chromiumplatings on light metals, such as, aluminum, :lluminum base alloys,magnesium and magnesium base oys.

Aluminium and aluminium alloys have hitherto been hard-chromium platedby first pickling the aluminimn or aluminium alloy. Subsequently aninterlayer was applied, in a variety of ways, which, in the case of someprocesses, was again dissolved away in a separate operation beforeentering the metal into the chromium plating bath, or in other processesserved as a bonding layer for the hard-chromium layer.

For example, in the production of electro-plated metals, an electrolyticflash precipitate was applied in a preliminary bath as a bonding layer,on to which the desired metal layer was applied in the electro-platingbath proper (c. f. Germany patent specification No. 242,142).Alternately a metallic interlayer was applied by cathodic sputtering, onwhich interlayer the hard metal layer, of for example, chromium, nickelor the like, was produced by electrolysis (c. f. German patentspecification No. 703,588).

In other processes, a layer of copper was precipitated onaluminium-magnesium alloys in a bath of copper chloride solution, whichlayer was then dissolved by nitric acid so as to roughen the surface ofthe alloy (c. f. German patent specification No. 762,716). It is alsoknown to produce a finely divided heavy metal covering layer onaluminium alloys which covering, after treatment with an acid metallicsalt solution is then removed with the aid of nitric acid, so that thesurface of the alloy is exposed (c. f. German patent specification No.760,230).

It has also been suggested to produce thick deposits of hard chromium onaluminium or aluminium alloys by first pre-treating said aluminium oraluminium alloy in an alkaline solution containing iron and zinc, thenremoving the resultant layer by means of nitric acid or a mixture ofsulphuric acid and hydrogen peroxide, subsequently applying a copperlayer of at the most mu by electrolysis in an alkaline copper platingbath and finally carrying out the chromium plating in a normal chromiumplating bath in accordance with known processes.

These known processes have the disadvantage that they are generallyapplicable only to certain groups of alloys and that hard chromiumlayers electrolytically applied according to these processes areinsufliciently bonded to the base metal especially when subjected tosimultaneous thermal and mechanical stresses. The metallic layer appliedbefore chromium plating is either removed in a separate process beforeimmersion of the metal into the chromium plating bath, so that thealuminium surface is not protected from oxidation up to the time ofapplication of the hard chromium layer, or the preliminarily appliedlayers are intended to serve as bonding layers and are preserved asinterlayers m such cases.

It was now found that the hard chromium layer can be very intensivelybonded to the base metal, i. e. light metal or light metal alloy,especially aluminium or aluminium alloy, if a preliminary metallic layeris ap plied electrolytically in such thickness that it is at leastalmost completely dissolved in the chromium-plating bath itself beforea. hard chromium layer has been formed. This can, for example, beeffected by a short reversal of the polarity before the chromium platingoperation.

These preliminary metallic layers enable the prevention of oxidationbefore immersion in the chromium plating bath. Brass, nickel, iron andcopper, for example, are suitable for such layers, especially when acomplete removal, advantageously by means of a short change of polarityof the cathode, lasting from 1 to 2 seconds, is produced before thefirst surge of plating current of the hard chromium plating process.This complete removal of the preliminary layer is obtained withpreliminary metal layers of a thickness of preferably 1 mu within 1 to 2seconds.

The hard chromium layers obtained in accordance with the presentinvention, are distinguished by a particularly intense bonding to thebase material and do not peel ofi even when strongly deformed.

The invention will be illustrated by the following nonlimitativeexample:

Example The surface of an aluminium or aluminium alloy workpiece isfirst pickled in accordance with a normal pre-treating process beforethe preliminary metallic layer is applied to the surface to be chromiumplated. This pre-treatment can, for example be effected by using a knownpickling solution containing ferrous or ferric chloride orboron-fiuorine-nickel chloride. The Workpiece to be plated is thenimmersed, for example in a brass plating bath under current, and isgalvauically treated until an electrolytic layer of brass of a thicknessof about 1 mu is formed. After rinsing with Water for a short time, theworkpiece is immersed in a chromium plating bath and treated anodicallyfor about 2 seconds until the preliminary layer has been dissolved away,whereupon the polarity is reversed. The workpiece is then hard chromiumplated under normal conditions using a current density of about 60 amps.per sq. decimetre.

What I claim is:

1. A method of applying a strongly adherent hard chromium plating uponan article having a surface of a light metal selected from the groupconsisting of aluminum, aluminum base alloys, magnesium and magnesiumbase alloys which comprises pickling the light metal surface to removeoxides, plating the thus cleaned surface with a thin metal layer of ametal selected from the group consisting of brass, nickel, iron andcopper, immersing the plated metal surface into a chromium plating bath,substantially completely dissolving said metal layer from said lightmetal surface while immersed in said chromium plating bath and thenimmediately electroplating the surface with hard chromium while still insaid chromium plating bath.

2. A method of applying a strongly adherent hard chromium plating uponan article having a surface of aluminum base metal which comprisespickling the aluminurn base metal surface to remove oxides, plating thethus cleaned surface with a thin metal layer of a metal selected fromthe group consisting of brass, nickel, iron and copper, immersing theplated metal surface into a chromium plating bath, substantiallycompletely dissolving said metal layer from said aluminum base metalsurface while immersed in said chromium plating bath and thenimmediately electroplating the surface with hard chromium while still insaid chromium plating bath.

3. A method of applying a strongly adherent hard chromium plating uponan article having a surface of a light metal selected from the groupconsisting of aluminum, aluminum base alloys, magnesium and magnesiumbase alloys which comprises pickling the light metal surface to removeoxides, plating the thus cleaned surface with a thin metal layer of ametal selected from the group consisting of brass, nickel, iron andcopper about l mu thick, immersing the plated metal surface into achromium plating bath, substantially completely dissolving said metallayer from said light metal surface while immersed in said chromiumplating bath and then immediately electroplating the surface with11313.31] chromium while still in said chromium plating 4. A method ofapplying a strongly adherent hard chromium plating upon an articlehaving a surface of a light metal selected from the group consisting ofaluminum, aluminum base alloys, magnesium and magnesium base alloyswhich comprises pickling the light metal surface to remove oxides,plating the thus cleaned surface with a thin layer of brass, immersingthe plated metal surface into a chromium plating bath, substantiallycompletely dissolving said metal layer from said light metal surfacewhile immersed in said chromium plating bath and then immediatelyelectroplating the surface with hard chromium while still in saidchromium plating bath.

5. A method of applying a strongly adherent hard chromium plating uponan article having a surface of a light metal selected from the groupconsisting of aluminum, aluminum base alloys, magnesium and magnesiumbase alloys which comprises pickling the light metal surface to removeoxides, plating the thus cleaned surface with a thin metal layer of ametal selected from the group consisting of brass, nickel, iron andcopper, immersing the plated metal surface into a chromium plating bath,substantially completely dissolving said metal layer from said lightmetal surface anodically while immersed in said chromium plating bathand then immediately electroplating the surface with hard chromium whilestill in said chromium plating bath.

6. A method of applying a strongly adherent hard chromium plating uponan article having a surface of a light metal selected from the groupconsisting of aluminum, aluminum base alloys, magnesium and magnesiumbase alloys which comprises pickling the light metal surface to removeoxides, plating the thus cleaned surface with a thin metal layer of ametal selected from the group consisting of brass, nickel, iron andcopper about l mu thick, immersing the plated metal surface into achromium plating bath, substantially completely dissolving said metallayer from said light metal surface anodically while immersed in saidchromium plating bath and then immediately electroplating the surfacewith hard chromium while still in said chromium plating bath.

7. A method of applying a strongly adherent hard chromium plating uponan article having a surface of a light metal selected from the groupconsisting of aluminum, aluminum base alloys, magnesium and magnesiumbase alloys which comprises pickling the light metal surface to removeoxides, plating the thus cleaned surface with a thin layer of brass,immersing the plated metal surface into a chromium plating bath,substantially completely dissolving said metal layer from said lightmetal surface anodically while immersed in said chromium plating bathand then immediately electroplating the surface with hard chromium Whilestill in said chromium plating bath.

8. A method of applying a strongly adherent hard chromium plating uponan article having a surface of a light metal selected from the groupconsisting of aluminum, aluminum base alloys, magnesium and magnesiumbase alloys which comprises pickling the light metal surface to removeoxides, plating the thus cleaned surface with a thin layer of brassabout l mu thick, immersing the plated metal surface into a chromiumplating bath, substantially completely dissolving said metal layer fromsaid light metal surface anodically While immersed in said chromiumplating bath and then immediately electroplating the surface with hardchromium while still in said chromium plating bath.

References Cited in the file of this patent UNITED STATES PATENTS2,091,386 Viers Aug. 31, 1937 2,327,676 Spence Aug. 24, 1943 FOREIGNPATENTS 258,724 Great Britain Sept. 30, 1926 476,720 Great Britain Dec.14, 1937 OTHER REFERENCES Metal Finishing, February 1949, pages 51 to53.

1. A METHOD OF APPLYING A STRONGLY ADHERENT HARD CHROMIUM PLATING UPONAN ARTICLE HAVING A SURFACE OF A LIGHT METAL SELECTED FROM THE GROUPCONSISTING OF ALUMINUM, ALUMINUM BASE ALLOYS, MAGNESIUM AND MAGNESIUMBASE ALLOYS WHICH COMPRISES PICKLING AND LIGHT METAL SURFACE TO REMOVEOXIDES, PLATING THE THUS CLEANED SURFACE WITH A THIN METAL LAYER OF AMETAL SELECTED FROM THE GROUP CONSISTING OF BRASS, NICKEL, IRON ANDCOPPER, IMMERSING THE PLATED METAL SURFACE INTO A CHROMIUM PLATING BATH,SUBSTANTIALLY COMPLETELY DIS SOLVING SAID METAL LAYER FROM SAID LIGHTMETAL SURFACE WHILE IMMERSED IN SAID CHROMIUM PLATING BATH AND THENIMMEDIATELY ELECTROPLATING THE SURFACE WITH HARD CHROMIUM WHILE STILL INSAID CHROMIUM PLATING BATH.